Conventional methods for developing electrostatic latent images by an electrophotographic method include the cascade phenomenon method, the touch down method, and the jumping method, etc. The cascade developing method disclosed in U.S. Pat. No. 3,105,770 involves sprinkling developing powder directly on a photoconductor. The cascade developing method was the first electrophotographic method applied to copying machines for practical use. Also, U.S. Pat. No. 3,866,574 discloses the developing method of stirring up one component toner by applying a.c. bias to a developing roller. In this method, the a.c. bias is applied so as to activate the movement of the toner, so that the toner is stirred up at image areas and returned at non-image areas on the photoconductor.
The method which improved the technique of applying the a.c. bias is the jumping developing method disclosed in Published Examined (Kokoku) Japanese Patent Application No. Sho 63-42256. In this method, the toner is supported by a toner support member, and on the toner support member there is provided a doctor blade for regulation of a rigid body or elastic body at a minute spacing to the support member. The toner is regulated into a thin layer by the doctor blade and transferred to a developing section, where the toner is deposited on the image areas of the photoconductor with the a.c. bias application. This method is different from the one disclosed in the above-mentioned U.S. Pat. No. 3,866,574 since the toner in the former method moves reciprocatingly between the image section and the non-image section.
It is well known that toner used for electrostatic developing methods including the methods mentioned above generally consists of resin, a coloring component such as pigment and dye, and an additive such as plasticizer and a charge control agent. As resin, natural or synthetic resin, or the combination of both is used.
The cascade developing method is poor in reproducing solid images. The method also requires an extremely large and complicated device. Moreover, the developing device disclosed in U.S. Pat. No. 3,866,574 requires high precision, and is complicated and costly. In the jumping developing method, a thin layer of the toner on a toner support member at a uniform thickness must always be formed. In addition, a previous image remains on the toner thin film, thus a residual image appears on an image in this method (sleeve ghost). There is also a problem in that the device used in this method is complicated and costly.
In order to solve these problems, an electrophotographic method disclosed in Published Unexamined (Kokai) Japanese Patent Application No. Hei 5-72890 was proposed. The device used in this method consists of a photoconductor containing a stationary magnet and an electrode roller having a magnet. The electrode roller faces the photoconductor with a predetermined gap in between. Thus, in this method, solid images are steadily reproduced and sleeve ghosts are not generated. Also, the device is further miniaturized and simplified, thus lowering the cost.
However, in order to improve the quality of images with this method, toner is required to be high in quality. In this method, since the doctor blade is not used, the toner is carried to a developing field between the photoconductor and the electrode roller without being controlled to a thin layer. Therefore, there is little space for the toner to be tribo-charged and to obtain tribo-charge amount, and the toner is required to have high chargeable properties and fluidity.
With a conventional toner which is used for a one-component developing method or a two-component developing method, images become uneven and fog in non-image sections increases. This is because the toner has low fluidity. A preferable level of tribo-charge cannot be obtained from a toner having low fluidity since the rate of contact with a developing member is low. Also, the tribo-charge becomes uneven within the toner, so the chargeable properties of the toner become uneven.
In order to increase the fluidity of toner, a method of adding silica, etc. as an additive is disclosed in Published Examined (Kokoku) Japanese Patent Application No. Sho 54-16219, and a method of using hydrophobic silica fine powder is disclosed in Published Unexamined (Kokai) Japanese Patent Applications No. Sho 46-5782, No. Sho 48-47345 and No. Sho 48-47346. For example, hydrophobic silica fine powder is prepared by reacting silica fine powder and an organic silicon compound such as dimethyl dichlorosilane, and replacing silanol groups on the surface of silica fine powder with organic groups. Although the fluidity of toner increases due to the additive, silica fine particles are likely to aggregate with each other. As a result, the suspended matter of silica increases, and a photoconductor will be scratched by the suspended matter. Residual films from the silica and toner are also generated on the photoconductor.
When using magnetic toner in which magnetic particles are contained as an internal additive, the particles are exposed after magnetic toner materials are pulverized. Thus, the toner will scratch a photoconductor, thus generating a film. With the film formed on a photoconductor, the surface potential of the photoconductor is not likely to decline when a charged photoconductor is exposed to light. As a result, in a reverse development, image defects such as the formation of white sections in a black image are found. White point noise is also generated since the suspended matter of silica adheres to a black image section. Thus, the addition of silica fine powder provides the above-noted additional problems.
In the electrophotographic method to which the magnetic toner of the invention is applied, the toner is first sprinkled over the entire surface of a photoconductor, and then developed. Therefore, compared with other conventional methods, toner is in contact with the photoconductor for a long time. As a result, toner film is likely to generate.
In order to prevent such a film, a friction reducing material such as polyvinylidene fluoride powder is disclosed in Published Examined (Kokoku) Japanese Patent Applications No. Sho 48-8136, No. Sho 48-8141 and No. Sho 51-1130.
Furthermore, Published Unexamined (Kokai) Japanese Patent Application No. Sho 48-47345 discloses the addition of a friction reducing material and an abrasive material in magnetic toner. Even though the addition is effective for eliminating toner film, paper dust, which adheres to a photoconductor surface due to repeated use, and low electrical resistance materials such as ozone products cannot be removed. The electrostatic latent image of the photoconductor thus is heavily damaged particularly in high temperature and humidity.
Published Unexamined (Kokai) Japanese Patent Applications No. Sho 60-32060 and No. Sho 59-219754 disclose the addition of titanate-based fine powder to toner as a second additive. The powder is mechanically pulverized, and the particle shape of the powder is irregular. Although the powder can be used to remove foreign matter on a photoconductor, protruding sections of the particles harm the photoconductor, thus distorting images. Moreover, in the electrophotographic method to which the magnetic toner of the invention is applied, toner is in contact with the entire surface of a photoconductor. Thus, when the titanate fine powder is simply added and copies with low black area ratios are taken, only the powder in the toner is consumed and used up in the long term, thus eliminating the ability of the toner to resist filming.
Also, in the method, a transferring roller is in contact with a photoconductor. Therefore, the abrasive material, friction-reducing material, etc. are transferred to the roller and are not supplied to a cleaning blade if the abrasive material, friction-reducing material, etc. are simply added to the toner. As a result, film cannot be prevented.
Thus merely adding other abrasive materials such as alumina and titania to magnetic toner provides a negative effect on the chargeable properties of the toner. As a result, image density is reduced, and fog increases.
Environmental protection has recently been an issue of great concern. In conventional copying machines, laser printers, laser plain paper facsimiles, etc., toner is developed on a photoconductor in the developing step, and the toner is then transferred to paper in a transferring step. Some of the toner remains on the photoconductor, and that toner is removed in a cleaning step. The cleaned toner, however, is residual toner. In conventional methods, particularly in the one-component developing method, the residual toner is not recycled.
A problem with recycling the residual toner is that the fluidity of the toner declines due to the stress received in a developing field, thus fluctuating charge amount. The residual toner with reduced fluidity aggregates and clogs up a doctor blade. When the residual toner of a conventional magnetic toner is recycled and mixed with new toner in a developing device, the charging amount distribution of the toner becomes uneven, and wrong sign toner.
Also, in order to recycle the residual toner for development, the toner has to be useful for a long period. In particular, the ability of the toner to resist against filming needs to be increased from the conventional level. Thus, improved dispersion of additives in the toner, reduced aggregation of the toner, and even adherence of the toner should be satisfied.
In the electrophotographic method to which the magnetic toner of the invention is applied, a conductive elastic roller is used. When a roller and conventional toner are used, letters and lines are transferred without the transfer of their internal image sections. Also, the toner is scattered around the letters and lines.
When conventional magnetic toner is transferred to transfer paper by a transfer roller, the roller is in contact with a photoconductor with predetermined stress. Compared with sections where there is no toner, a lot of toner is deposited at sections where the toner is concentrated and the stress increases. As a result, the toner aggregates due to high stress, and is not transferred to the transfer paper. Therefore, letters and lines are transferred without the transfer of internal image sections (hollow characters). The toner on a photoconductor is transferred to transfer paper by the relation among the charge potential of toner, the opposite charge potential of roller added from outside. Therefore, when the potential of the toner is low, the toner scatters on the empty space of paper around the letters and lines.
When conventional magnetic toner is used, the toner is not recycled. Thus, natural resources are not effectively used, and the environment is harmed.